The present invention relates to the evaluation of magnetic data storage media either during or after fabrication, and more particularly to devices and processes for determining magnetic properties in the magnetizable recording layers of such media.
Disk drives employ one or more rotatable disks in combination with transducers supported for generally radial movement relative to the disks. Usually, each transducer is maintained spaced apart from its associated disk at a "flying height" controlled by an air bearing caused by disk rotation. Alternatively in connection with flexible disks, the transducer is part of a recording head maintained in sliding contact with the disk. In either event, each transducer interacts with a magnetizable recording layer of its associated disk--applying magnetic fields to write magnetic transitions (data) into the recording layer, sensing the resulting fringing fields to read the data, or applying erasing fields to remove data previously written.
Magnetic media performance depends largely upon magnetic properties of the recording layer. Important properties in this regard include the coercivity, the remanence-thickness product, and the coercive squareness or switching squareness. The coercivity is the magnetic intensity of an applied magnetic field sufficient to cause the recording layer to undergo a transition from a state of magnetic saturation to a non-magnetized state. The remanence-thickness product indicates the density of magnetic poles in the remanent state and thus relates to the strength of the electrical signal recoverable from a magnetization transition. The coercive squareness or switching squareness describes the rate of change of magnetization as a function of an applied magnetic field, in the vicinity of the coercivity and thus influences writing and transition formation.
There are several known approaches to testing magnetic media for their magnetic properties, including vibrating sample magnetometers for measuring magnetization as a function of an applied magnetic field, and hysteresis testers for measuring flux density as a function of an applied field. These devices, however, are destructive in that they require cutting or otherwise separating a finite element from the recording medium for measurement.
U.S. Pat. No. 4,847,558, assigned to the assignee of this application and incorporated herein by reference, discloses a non-destructive approach for determining coercive force (i.e. coercivity) and remanence-thickness product. In particular, a standard read/write head (magnetic induction transducer) supported on an air bearing, is used to generate currents of increasing strength and thereby apply erasing fields of increasing strength to previously recorded transitions. The magnetic intensities of the applied erase fields are calculated, based on an assumed linear current-field relationship, on the basis that the coercive force coincides with a negative 6 dB signal level, as compared to the signal level before applying the erase current. To determine remanence-thickness product, the isolated half-pulse width of the negative 6 dB signal is calculated.
While this approach offers considerable advantages as compared to destructive testing, several factors limit its utility.
The first of these is the assumption of a linear current-field relationship. The relationship holds true over only a limited range in any event, and its non-linearity is more pronounced when thin film heads and thin film magnetic media are involved.
Second, there are offsets in the current-field relationship due to head remanence.
Third, fluctuations or errors in transducer/disk spacing lead to uncertainties in determining the applied erasing field.
Therefore, it is an object of the present invention to provide a non-destructive process for determining remanence-thickness products of magnetic media by direct measurement of fringing fields from recorded transitions, without the need to determine an isolated pulse width.
Another object is to provide a system for measuring coercivities of magnetic media through direct measurement of erase fields applied to previously recorded transitions, avoiding the need to rely on an assumed erase current/erase field relationship.
A further object of the invention is to provide a device for applying magnetic erase fields to determine magnetic properties of data storage media, in a manner that substantially reduces the chance for errors due to variance in transducer/disk spacing.
Yet another object is to provide a rapid, non-destructive process for measuring magnetic properties and mapping such properties throughout the magnetizable recording layer of a data storage medium.